The need to conserve fossil fuels is only one reason why space heating, water heating and the like using the energy of the sun has received so much recent attention. For example, space heating utilizing solar energy flat-plate collectors is increasing in use. Typical solar collector systems utilize a flat-plate collector containing a metallic plate painted black, with one or two glass covers. The sides and bottom of the collector insulate the collector plate. About 90 percent of the solar energy transmitted through the glass and reaching the metallic plate is absorbed by the plate, increasing its temperature. The absorbed energy is, in turn, transferred to a working fluid--sometimes air or a liquid which is preferably (but not always) water.
If the working fluid is water, it is passed through tubes attached to the metallic plate. Water to be heated is pumped from a source to the tubing of the solar collector where the plate-absorbed energy is transferred thereto, with the water acting as the thermal energy storage material for the system. The heated water is then returned to a storage tank where it remains until used for space heating or as a potable (hot) water supply.
While such solar collector systems enjoy widespread use, they are not without certain disadvantages. One of the more important and debilitating problems occurs when the temperature of the solar collector drops to or below the freezing point of the working fluid. When this occurs, the expansion of the frozen working fluid (water, for example) can severely damage the communicating pipes and tubing of the solar collector and system.
Various solutions to this problem have heretofore been proposed and used to some advantage--but not without concomitant disadvantages. For example, air may be used as the working fluid of the solar collector system. However, this requires a second energy transfer in order to heat water for home consumption. Moreover, in addition to being a poor energy storage material, air requires a relatively high blower power for circulation and bulkier ducts and flow passages.
Another solution contemplated is use of an antifreeze mixture as the working fluid of sufficient concentration to preclude freezing under the most severe weather conditions reasonably expected in the locale in which the solar collector system is being used. Unfortunately, this solution also has a number of drawbacks. The antifreeze mixture can be expensive and subject to frequent replacement. Moreover, the longevity of many antifreeze chemicals is tied to the maximum surface temperature experienced. For example, propylene glycol cannot be used where temperatures may exceed 300.degree. F., a value which can easily be reached and exceeded by many presently known collectors under no flow or stagnation conditions. Thus, a working fluid including propylene glycol may avoid freeze damage for only a short period--or until the working temperatures seriously mitigate the (freeze protection) usefulness of the mixture. Further, an additional energy transfer must be made via a heat exchanger in order to isolate the antifreeze mixture from space heating or domestic water heating applications. The heat exchanger typically requires specialized and costly equipment (e.g., heat exchange bundle, expansion tank, secondary pressure relief, antifreeze solution).
Another method of protecting solar collector systems from freeze damage, and one to which this invention is directed, involves draining the collector array when freezing or near freezing conditions are detected. This technique (typically referred to as a "drain-down" technique) requires the collector array to be designed with suitable provisions for gravity drainage and air control for proper draining of the array and subsequent refilling. A number of advantages are obtained by this method over those discussed above. For example, if water is the working fluid (as it usually is), the need for heat exchanger equipment and antifreeze chemical treatment is avoided since the water can be circulated through the collector array for heating and used directly.
Solar collector systems utilizing this latter approach to freeze protection typically feature an electromechanical (e.g., solenoid) actuated valve in combination with a temperature sensing device to monitor the working fluid (water) contained in the collector array. When the temperature of working fluid in the collector drops to a first predetermined level, the sensor interrupts electrical current to the valve actuator, causing the actuator to interrupt communication of the water to and from the storage tank and to open a drain path for water contained in the array so that it may drain from the array before it freezes. When the ambient temperature rises to a second predetermined level--usually above the first--the sensor again communicates electrical current to the actuator to close the drain path and reopen communication between the array and the storage tank or other facility. Examples of this type of system may be seen in U.S. Pat. Nos. 3,812,872 and 4,044,754.
While this latter technique may be advantageous in many respects, it is not without certain undesirable difficulties. For example, valve malfunctions frequently occur due to particle contamination in flow and/or control parts. Such contamination will cause the valve to stick and fail to provide the necessary drain and flow control functions when needed. Furthermore, since the actuators are usually of a fast-action type (e.g., the valve is opened or closed in fractions of a second), components of the solar collector system can be severely damaged due to hydraulic surges or water hammering during fill operation.
Moreover, many drain-down schemes employing electromechanically operated valves utilize significant quantities of electrical energy. This problem is magnified by the requirement for "fail-safe" draining operation in the event of medium or long-term power outage. To achieve a fail-safe drain-down capability, the apparatus must be preloaded to return to a drain configuration when the power is lost. In practice, this requires that certain components be continually energized (e.g., in a standby state). Thus, while the operational period of the solar collection equipment is limited to daylight hours, the drain-down apparatus will require standby power on a twenty-four hour basis. Under these conditions, the electrical energy consumption of conventional electromechanical valves can become significant when compared to the energy savings afforded by the collector array.
Drain-down configurations using electromechanically operated valves are also prone to "false" drain-down cycles caused by momentary power losses on the order of seconds or minutes. Such behavior unnecessarily stresses ancillary drain-down components such as the automatic air vents.
Too, drain-down schemes comprised of discrete electromechanical valves often result in a collector filling operation (after draining) which introduces water to one leg of the collector array only. This is particularly true of systems which incorporate a check valve to both prevent reverse circulation at night and to serve as a positive isolation between the collector array and source pressure in the drain-down mode. Such a filling procedure frequently leads to the entrapment of air in the communicating lines of the system.